In wireless cellular networks, the deployment of fixed relay stations has been considered as an alternative for improving not only cell coverage but also transmission efficiency. Recent studies have focused on the analysis of adequate deployment scenarios, taking into account aspects such as the cost/efficiency trade-off of relay stations and resource allocation issues. Bi-directional communication is considered, in which a Mobile Station (MS) and a Base Station (BS) communicate with each other through a Relay Station (RS).
FIG. 1 depicts a general system model of a wireless communication system 100 that includes a BS 110 having a plurality of antennas 112, an RS 120 having another plurality of antennas 122, and a plurality of MSs 130-1, 130-2, . . . , 130-N. Each of the N MSs has a single antenna. As depicted in FIG. 1, the MSs, or users, cannot connect directly to the BS, e.g., because they are in a situation of strong shadow fading, and so the MSs and BS use two-hop communication through the RS. The artisan will recognize that a typical wireless network includes many base stations and can include many relay stations.
Networks such as the example depicted in FIG. 1 have been studied. For example, P. Popovski and H. Yomo, “Bi-Directional Amplification of Throughput in a Wireless Multi-Hop Network”, Proc. IEEE Vehicular Technology Conf. (VTC), vol. 2, pp. 588-593 (May 2006) and T. Unger and A. Klein, “Duplex Schemes in Multiple Antenna Two-Hop Relaying”, EURASIP J. Advances in Signal Processing, vol. 2008, pp. 1-14 (2008) consider cases where there is a single MS/BS pair communicating through an RS, or more recently with multiple pairs sharing a same relay.
Assuming the availability of one or more relay stations within a cell that enable two-hop communication between MS/BS pairs, bi-directional relaying has appeared to be a way to improve system performance. Bi-directional relaying can make it possible to implement both downlink and uplink communication with the least amount of resources.
Previous work has investigated bi-directional relaying techniques employing both Amplify-and-Forward (AF) and Decode-and-Forward (DF) techniques. FIG. 2 schematically depicts one-way AF relaying transmission from a first node i to a second node j through the relay station r. It will be understood from FIG. 1 that the nodes i, j can be any of the MSs 130 and the BS 110. A wireless signal transmitted by the node i during a first time period, which can be called a time slot, typically includes one or more information symbols si,j intended for the node j. The wireless signal transmitted by the node i is modified by passage through a communication channel to the relay station r according to the channel's impulse response hi,r. During a succeeding time slot, the relay station r transmits a signal that can be denoted yr to the node j, and that signal is modified by passage through another communication channel to the node j according to the channel's impulse response hr,j. It will be understood that the reverse-direction communication from node j to node i will take two further time slots.
In order to provide bi-directional communication, either self-interference cancellation (for AF relaying) or network coding (for DF relaying) can be employed. The above-cited publication by Popovski et al. and U.S. Pat. No. 7,336,930 to Larsson et al. for “Interference Cancellation in Wireless Relaying Networks” (Feb. 26, 2008) describe uses of self-interference cancellation. U.S. Patent Application Publication No. US 2009/0268662A1 by P. Larsson, N. Johansson, and K. Sunell for “Method and Arrangement for Bi-Directional Relaying in Wireless Communication Systems” (Oct. 29, 2009) and P. Larsson, N. Johansson, and K.-E. Sunell, “Coded Bi-Directional Relaying”, Proc. IEEE VTC, vol. 2, pp. 851-855 (May 2006) describe uses of network coding. In addition, through the use of multi-antenna network elements, it is also possible to implement bi-directional communication, such as described in the above-cited publication by Unger et al.
A problem with existing implementations of two-hop bi-directional communication in a network such as depicted in FIG. 1 is that the number of antennas required at the RS is exceedingly high, if simultaneous transmission of all MS-BS pairs is desired, or the time required to complete communication can be exceedingly long, if the MS-BS pairs are sequentially served. In order to bi-directionally serve N MSs simultaneously, an N-antenna BS and a 2N-antenna RS are required. Since it is desirable for an RS to be a low-cost version of a BS, an RS with so many more antennas than the BS can preclude use of relaying in practical networks.